1. Field of the Invention
The invention relates to a method for magnetic resonance imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field, the method comprising applying an excitation radio-frequency pulse (RF-pulse) for excitation of nuclear dipole moments in said portion of the body, followed by switching a plurality of magnetic field read gradient pulses having alternate positive and negative polarity for generating a plurality of magnetic resonance signals in the excited portion, and simultaneously measuring signal samples of said magnetic resonance signals and using said acquired signal samples for the reconstruction of an image of said body. The invention also relates to an apparatus for performing such a method.
2. Description of the Related Art
In such a method the acquired signal samples have phase errors which are mainly due to the fact that the positive and negative polarity magnetic field read gradient pulses are subject to more differences than only their polarity. Due to differences in hardware, switching history, eddy current effects in the MRI device, etcetera, the shape and time integrated size of the pulses varies during a measurement sequence. Consequently, phase errors occur in the signals samples which phase errors appear polarity dependent. In the European Patent Application No. 93202678.4, filed 16 September 1993 and forming prior art under Article 54(3) EPC with respect to the European counter part of the application, a method according to the introductory paragraph is described. According to this prior method the phase errors are determined in an area of k-space as a function of the phase-encoding gradient strength. Subsequently, the obtained phase errors are extrapolated over the whole of k-space and taken into account during the image reconstruction procedure. In this way an image is obtained in which the effects of the difference of the alternating polarity of the magnetic field read gradients is largely removed.
In the method according to the introductory paragraph, imaging errors are not only caused by the alternation of the magnetic field read gradient, another cause of imaging errors is the transverse relaxation of the magnetization in the body. Due to this so-called T.sub.2 -relaxation the signal size for magnetic resonance signals decreases with increasing time from the excitation RF-pulse. The relaxation time constant T.sub.2 is different for different types of matter, i.e. different types of tissue in a patient investigated by magnetic resonance imaging. As signals from different regions in k-space are measured at different time intervals from the excitation RF-pulse, a different weight is applied to the many measured signal samples in k-space. In particular if k-space is scanned such that the phase-encoding value is not a monotonic function of the time to the excitation RF-pulse, this will lead to ghost-artefacts.